On the southeastern shore of Bolivia's Lake Titicaca stand the ancient ruins of the city of Tiwanaku. Here, the remains of monolithic rock sculptures and buildings over 1,500 years old can be seen. What makes this site so incredible is not merely the size and weight of the stone used by these pre-Inca builders, but the fact that much of that stone originated far from the ruins - around 55 miles (90 kilometers) across the wide blue lake!